Technical Field
The present invention relates to monitoring circuit degradation and, more particularly, to degradation sensors that monitor both hot carrier injection degradation and bias-temperature instability degradation of semiconductor devices and structures.
Description of the Related Art
There are several physical mechanisms that cause complementary metal-oxide semiconductor (CMOS) field effect transistor (FET) degradation over time. As the circuit composed of CMOS FETs ages and is used, these degradation mechanisms can cause important characteristics of the circuit (such as, e.g., threshold voltage) to shift or can eventually cause catastrophic failure of the circuit.
One mechanism of degradation is bias-temperature instability (BTI). BTI produces increases in the threshold voltage of a semiconductor device and reduces the drain current of the device. The progression of BTI degradation is determined by total power-on time of the device and is a result of charge carriers tunneling through barriers and damaging the material along interfaces. A second degradation mechanism is hot carrier injection (HCI). In contrast, HCI degradation typically occurs when the device switches states. HCI is caused by “hot” electrons that have enough energy to become trapped in the gate dielectric or to create interface traps. In both cases, BTI and HCI cause physical damage to the device, degrade the properties of the device, such that the circuit may eventually fail.
Referring now to FIG. 1, a comparison between the progression of BTI and HCI degradation is shown. The vertical axis represents the logarithm of the degradation that has accumulated, while the horizontal axis is the logarithm of the time elapsed. The BTI curve has a rough time dependence of about t0.2, while the HCI curve 104 has a time dependence of about t0.5. HCI is a more dramatic cause of degradation at longer time spans. As shown in FIG. 1, the curve 106 becomes the dominant component of the degradation after a certain point in time.
However, existing on-chip degradation monitors are largely insensitive to HCI degradation. As a result, they may not detect degradation due to HCI effects until after other parts of the circuit have already undergone significant degradation or failure. As a result, such monitors are not able to detect the rapid aging that occurs at long timespans.